The present invention relates to a novel resin composition. More particularly, the present invention relates to a novel soft polypropylene resin composition having flexibility at normal temperature and a sufficient mechanical strength at high temperatures.
In recent years, soft vinyl chloride resins containing a plasticizer have been widely used as a soft resin for sheets or films. Soft vinyl chloride resins, however, may give rise to social problems such as (1) toxicity problem caused by bleed-out of plasticizer or monomer used therein and (2) acid rain derived from hydrogen chloride generated by burning thereof.
Meanwhile, as a soft resin comparable to the soft vinyl chloride resins, there are resins using ethylene as a main component, such as ethylene/vinyl acetate copolymer, ethylene/ethyl acrylate copolymer, ionomer, linear low-density polyethylene, ultralow-density polyethylene, ethylene/propylene copolymer and the like. These ethylene- based soft resins, however, are not balanced in flexibility and mechanical strength. That is, resins having sufficient flexibility at normal temperature tend to have a low mechanical strength at about 80.degree. C., while resins having an excellent mechanical strength at about 8.degree. C. have low flexibility at normal temperature.
It was attempted to allow a polypropylene resin inherently having an excellent mechanical strength to have flexibility by copolymerizing propylene with ethylene to obtain a propylene/ethylene copolymer having a lower melting point, a sufficient mechanical strength and flexibility. In this approach, however, the ethylene content is at best 5-6% by weight under the present technique; hence, it is difficult to obtain a polypropylene having flexibility comparable to that of soft vinyl chloride resins.
In view of the above situation, an object of the present invention is to provide a soft resin composition other than soft vinyl chloride resins, in particular, a resin composition having excellent flexibility and mechanical strength over a temperature range from normal temperature to high temperatures.
Other object of the present invention is to provide a resin composition which has a low cost and which can be suitably used in various applications, for example, as a packaging film or sheet, a building material sheet in the construction field, a carpet backing in the automobile and construction fields, an insulator for cable, a fiber, and a base material for tape.